How to make all edges in the scene rendered by freestyle?
A simple python script can turn them all on/off

import bpy

for object in bpy.data.objects:
    if object.type == 'MESH':
        for edge in object.data.edges:
            edge.use_freestyle_mark = True

        #show the marked edges
        object.data.show_freestyle_edge_marks = True


Just paste this into the text editor, and press the hotkey ALTP, or the Run Script button to run it.

Then, to remove the marks, just change True's to False, and run the script and they should all be gone.